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NAME

       tcp - TCP protocol

SYNOPSIS

       #include <sys/socket.h>
       #include <netinet/in.h>
       #include <netinet/tcp.h>
       tcp_socket = socket(PF_INET, SOCK_STREAM, 0);

DESCRIPTION

       This  is  an  implementation  of  the  TCP protocol defined in RFC 793,
       RFC 1122 and  RFC 2001  with  the  NewReno  and  SACK  extensions.   It
       provides  a  reliable,  stream-oriented, full-duplex connection between
       two sockets on top  of  ip(7),  for  both  v4  and  v6  versions.   TCP
       guarantees that the data arrives in order and retransmits lost packets.
       It generates and checks a per-packet  checksum  to  catch  transmission
       errors.  TCP does not preserve record boundaries.

       A  newly  created  TCP socket has no remote or local address and is not
       fully specified.  To create an outgoing TCP connection  use  connect(2)
       to  establish  a  connection  to  another  TCP  socket.  To receive new
       incoming connections, first bind(2) the socket to a local  address  and
       port  and  then  call  listen(2)  to  put the socket into the listening
       state.  After that a new socket for each  incoming  connection  can  be
       accepted using accept(2).  A socket which has had accept() or connect()
       successfully called on it is fully specified  and  may  transmit  data.
       Data cannot be transmitted on listening or not yet connected sockets.

       Linux supports RFC 1323 TCP high performance extensions.  These include
       Protection Against Wrapped Sequence Numbers (PAWS), Window Scaling  and
       Timestamps.  Window scaling allows the use of large (> 64K) TCP windows
       in order to support links with high latency or bandwidth.  To make  use
       of them, the send and receive buffer sizes must be increased.  They can
       be set globally with the net.ipv4.tcp_wmem and net.ipv4.tcp_rmem sysctl
       variables,  or  on  individual  sockets  by  using  the  SO_SNDBUF  and
       SO_RCVBUF socket options with the setsockopt(2) call.

       The maximum sizes for socket buffers declared  via  the  SO_SNDBUF  and
       SO_RCVBUF  mechanisms  are  limited by the global net.core.rmem_max and
       net.core.wmem_max sysctls.  Note that TCP actually allocates twice  the
       size  of  the  buffer  requested  in  the  setsockopt(2) call, and so a
       succeeding getsockopt(2) call will not return the same size  of  buffer
       as  requested  in the setsockopt(2) call.  TCP uses the extra space for
       administrative purposes and internal kernel structures, and the  sysctl
       variables  reflect the larger sizes compared to the actual TCP windows.
       On individual connections, the socket buffer size must be set prior  to
       the  listen()  or  connect() calls in order to have it take effect. See
       socket(7) for more information.

       TCP supports urgent data.  Urgent data is used to signal  the  receiver
       that  some  important  message  is  part of the data stream and that it
       should be processed as soon as possible.  To send urgent  data  specify
       the  MSG_OOB  option  to  send(2).   When  urgent data is received, the
       kernel sends a SIGURG signal to the process or process group  that  has
       been  set as the socket "owner" using the SIOCSPGRP or FIOSETOWN ioctls
       (or  the  SUSv3-specified  fcntl(2)  F_SETOWN  operation).   When   the
       SO_OOBINLINE  socket  option  is  enabled,  urgent data is put into the
       normal data stream (a program can  test  for  its  location  using  the
       SIOCATMARK  ioctl  described  below), otherwise it can be only received
       when the MSG_OOB flag is set for recv(2) or recvmsg(2).

       Linux 2.4 introduced a number of changes for  improved  throughput  and
       scaling,  as  well  as  enhanced functionality.  Some of these features
       include  support  for  zero-copy   sendfile(2),   Explicit   Congestion
       Notification,  new  management  of TIME_WAIT sockets, keep-alive socket
       options and support for Duplicate SACK extensions.

ADDRESS FORMATS

       TCP is built on top of IP (see ip(7)).  The address formats defined  by
       ip(7)  apply  to  TCP.  TCP only supports point-to-point communication;
       broadcasting and multicasting are not supported.

SYSCTLS

       These variables can be accessed by the  /proc/sys/net/ipv4/*  files  or
       with  the sysctl(2) interface.  In addition, most IP sysctls also apply
       to TCP; see ip(7).  Variables described  as  Boolean  take  an  integer
       value,  with  a  non-zero value ("true") meaning that the corresponding
       option is enabled, and a zero value ("false") meaning that  the  option
       is disabled.

       tcp_abort_on_overflow (Boolean; default: disabled)
              Enable  resetting  connections  if  the listening service is too
              slow and unable to keep up and accept them.  It  means  that  if
              overflow  occurred  due to a burst, the connection will recover.
              Enable this  option  only  if  you  are  really  sure  that  the
              listening  daemon  cannot be tuned to accept connections faster.
              Enabling this option can harm the clients of your server.

       tcp_adv_win_scale (integer; default: 2)
              Count  buffering  overhead  as   bytes/2^tcp_adv_win_scale   (if
              tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale), if
              it is <= 0.

              The  socket  receive  buffer  space  is   shared   between   the
              application and kernel.  TCP maintains part of the buffer as the
              TCP window, this is the size of the receive window advertised to
              the   other  end.   The  rest  of  the  space  is  used  as  the
              "application"  buffer,  used  to  isolate   the   network   from
              scheduling  and  application  latencies.   The tcp_adv_win_scale
              default  value  of  2  implies  that  the  space  used  for  the
              application buffer is one fourth that of the total.

       tcp_app_win (integer; default: 31)
              This  variable  defines  how  many  bytes  of the TCP window are
              reserved for buffering overhead.

              A maximum of (window/2^tcp_app_win, mss) bytes in the window are
              reserved  for the application buffer.  A value of 0 implies that
              no amount is reserved.

       tcp_bic (Boolean; default: disabled)
              Enable BIC TCP  congestion  control  algorithm.   BIC-TCP  is  a
              sender-side only change that ensures a linear RTT fairness under
              large windows while offering both scalability and  bounded  TCP-
              friendliness.  The protocol combines two schemes called additive
              increase and binary search increase. When the congestion  window
              is  large,  additive  increase  with  a  large increment ensures
              linear RTT fairness as well as  good  scalability.  Under  small
              congestion   windows,   binary   search  increase  provides  TCP
              friendliness.

       tcp_bic_low_window (integer; default: 14)
              Sets the threshold window (in packets) where BIC TCP  starts  to
              adjust  the  congestion  window.  Below  this  threshold BIC TCP
              behaves the same as the default TCP Reno.

       tcp_bic_fast_convergence (Boolean; default: enabled)
              Forces BIC TCP to more quickly respond to changes in  congestion
              window. Allows two flows sharing the same connection to converge
              more rapidly.

       tcp_dsack (Boolean; default: enabled)
              Enable RFC 2883 TCP Duplicate SACK support.

       tcp_ecn (Boolean; default: disabled)
              Enable RFC 2884 Explicit Congestion Notification.  When enabled,
              connectivity  to  some  destinations  could  be  affected due to
              older, misbehaving routers along the path causing connections to
              be dropped.

       tcp_fack (Boolean; default: enabled)
              Enable TCP Forward Acknowledgement support.

       tcp_fin_timeout (integer; default: 60)
              This  specifies  how many seconds to wait for a final FIN packet
              before the socket  is  forcibly  closed.   This  is  strictly  a
              violation  of  the  TCP  specification,  but required to prevent
              denial-of-service attacks.  In Linux 2.2, the default value  was
              180.

       tcp_frto (Boolean; default: disabled)
              Enables   F-RTO,   an   enhanced   recovery  algorithm  for  TCP
              retransmission  timeouts.   It  is  particularly  beneficial  in
              wireless  environments  where  packet  loss  is typically due to
              random  radio  interference  rather  than  intermediate   router
              congestion.

       tcp_keepalive_intvl (integer; default: 75)
              The number of seconds between TCP keep-alive probes.

       tcp_keepalive_probes (integer; default: 9)
              The  maximum  number  of  TCP  keep-alive  probes to send before
              giving up and killing the connection if no response is  obtained
              from the other end.

       tcp_keepalive_time (integer; default: 7200)
              The  number  of seconds a connection needs to be idle before TCP
              begins sending out keep-alive probes.  Keep-alives are only sent
              when  the  SO_KEEPALIVE  socket  option is enabled.  The default
              value  is  7200  seconds  (2  hours).   An  idle  connection  is
              terminated  after  approximately  an  additional  11  minutes (9
              probes an interval of  75  seconds  apart)  when  keep-alive  is
              enabled.

              Note   that   underlying   connection  tracking  mechanisms  and
              application timeouts may be much shorter.

       tcp_low_latency (Boolean; default: disabled)
              If enabled, the TCP stack  makes  decisions  that  prefer  lower
              latency  as  opposed  to  higher  throughput.  It this option is
              disabled, then higher throughput is preferred.  An example of an
              application  where  this  default  should  be changed would be a
              Beowulf compute cluster.

       tcp_max_orphans (integer; default: see below)
              The maximum number of orphaned (not attached to  any  user  file
              handle)  TCP sockets allowed in the system.  When this number is
              exceeded, the orphaned connection is  reset  and  a  warning  is
              printed.   This  limit  exists only to prevent simple denial-of-
              service  attacks.   Lowering  this  limit  is  not  recommended.
              Network  conditions  might require you to increase the number of
              orphans allowed, but note that each orphan can eat up to ~64K of
              unswappable  memory.   The default initial value is set equal to
              the kernel parameter NR_FILE.  This initial default is  adjusted
              depending on the memory in the system.

       tcp_max_syn_backlog (integer; default: see below)
              The  maximum  number  of  queued  connection requests which have
              still  not  received  an  acknowledgement  from  the  connecting
              client.   If  this  number  is  exceeded,  the kernel will begin
              dropping requests.  The default value of  256  is  increased  to
              1024  when  the  memory  present  in  the  system is adequate or
              greater (>= 128Mb), and reduced to 128 for  those  systems  with
              very low memory (<= 32Mb).  It is recommended that if this needs
              to be increased above 1024, TCP_SYNQ_HSIZE in  include/net/tcp.h
              be  modified to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog, and
              the kernel be recompiled.

       tcp_max_tw_buckets (integer; default: see below)
              The maximum number of sockets in TIME_WAIT state allowed in  the
              system.   This  limit  exists  only to prevent simple denial-of-
              service attacks.  The default value  of  NR_FILE*2  is  adjusted
              depending  on  the  memory  in  the  system.   If this number is
              exceeded, the socket is closed and a warning is printed.

       tcp_mem
              This is a vector of 3 integers: [low,  pressure,  high].   These
              bounds  are used by TCP to track its memory usage.  The defaults
              are calculated at boot time from the amount of available memory.

              low - TCP doesn’t regulate its memory allocation when the number
              of pages it has allocated globally is below this number.

              pressure - when the amount of memory allocated  by  TCP  exceeds
              this  number  of  pages,  TCP  moderates its memory consumption.
              This memory pressure state is exited once the  number  of  pages
              allocated falls below the low mark.

              high  -  the  maximum  number  of pages, globally, that TCP will
              allocate.  This value overrides any other limits imposed by  the
              kernel.

       tcp_orphan_retries (integer; default: 8)
              The  maximum number of attempts made to probe the other end of a
              connection which has been closed by our end.

       tcp_reordering (integer; default: 3)
              The maximum a packet can be reordered in  a  TCP  packet  stream
              without  TCP assuming packet loss and going into slow start.  It
              is not advisable to  change  this  number.   This  is  a  packet
              reordering  detection  metric  designed  to minimize unnecessary
              back off and retransmits provoked by reordering of packets on  a
              connection.

       tcp_retrans_collapse (Boolean; default: enabled)
              Try to send full-sized packets during retransmit.

       tcp_retries1 (integer; default: 3)
              The  number  of times TCP will attempt to retransmit a packet on
              an established connection normally, without the extra effort  of
              getting the network layers involved.  Once we exceed this number
              of retransmits, we first have the network layer update the route
              if  possible before each new retransmit.  The default is the RFC
              specified minimum of 3.

       tcp_retries2 (integer; default: 15)
              The maximum number of times a TCP  packet  is  retransmitted  in
              established  state  before  giving up.  The default value is 15,
              which corresponds to a duration of approximately between  13  to
              30  minutes,  depending  on  the  retransmission  timeout.   The
              RFC 1122 specified minimum limit of  100  seconds  is  typically
              deemed too short.

       tcp_rfc1337 (Boolean; default: disabled)
              Enable  TCP  behaviour conformant with RFC 1337.  When disabled,
              if a RST is received in TIME_WAIT state,  we  close  the  socket
              immediately without waiting for the end of the TIME_WAIT period.

       tcp_rmem
              This is a vector of 3  integers:  [min,  default,  max].   These
              parameters  are  used  by  TCP to regulate receive buffer sizes.
              TCP dynamically adjusts the size of the receive buffer from  the
              defaults  listed  below, in the range of these sysctl variables,
              depending on memory available in the system.

              min - minimum size of  the  receive  buffer  used  by  each  TCP
              socket.   The  default  value is 4K, and is lowered to PAGE_SIZE
              bytes in low-memory systems.  This value is used to ensure  that
              in  memory pressure mode, allocations below this size will still
              succeed.  This is not used to bound  the  size  of  the  receive
              buffer declared using SO_RCVBUF on a socket.

              default  -  the  default  size  of  the receive buffer for a TCP
              socket.  This value overwrites the initial default  buffer  size
              from  the  generic  global net.core.rmem_default defined for all
              protocols.  The default value is 87380 bytes, and is lowered  to
              43689 in low-memory systems.  If larger receive buffer sizes are
              desired, this value should be increased (to affect all sockets).
              To  employ  large  TCP  windows, the net.ipv4.tcp_window_scaling
              must be enabled (default).

              max - the maximum size of the receive buffer used  by  each  TCP
              socket.     This    value   does   not   override   the   global
              net.core.rmem_max.  This is not used to limit the  size  of  the
              receive  buffer  declared  using  SO_RCVBUF  on  a  socket.  The
              default value of 87380*2 bytes is lowered to 87380 in low-memory
              systems.

       tcp_sack (Boolean; default: enabled)
              Enable RFC 2018 TCP Selective Acknowledgements.

       tcp_stdurg (Boolean; default: disabled)
              If  this option is enabled, then use the RFC 1122 interpretation
              of  the   TCP   urgent-pointer   field.    According   to   this
              interpretation,  the  urgent  pointer points to the last byte of
              urgent data.  If this option is  disabled,  then  use  the  BSD-
              compatible  interpretation  of  the  urgent  pointer: the urgent
              pointer  points  to  the  first  byte  after  the  urgent  data.
              Enabling this option may lead to interoperability problems.

       tcp_synack_retries (integer; default: 5)
              The  maximum number of times a SYN/ACK segment for a passive TCP
              connection will be retransmitted.  This  number  should  not  be
              higher than 255.

       tcp_syncookies (Boolean)
              Enable  TCP  syncookies.   The  kernel  must  be  compiled  with
              CONFIG_SYN_COOKIES.  Send out syncookies when  the  syn  backlog
              queue of a socket overflows.  The syncookies feature attempts to
              protect a socket from a SYN flood attack.  This should  be  used
              as  a  last  resort,  if at all.  This is a violation of the TCP
              protocol, and conflicts with other areas  of  TCP  such  as  TCP
              extensions.   It  can cause problems for clients and relays.  It
              is not recommended as a  tuning  mechanism  for  heavily  loaded
              servers  to  help  with  overloaded or misconfigured conditions.
              For   recommended    alternatives    see    tcp_max_syn_backlog,
              tcp_synack_retries, and tcp_abort_on_overflow.

       tcp_syn_retries (integer; default: 5)
              The  maximum  number  of  times  initial  SYNs for an active TCP
              connection attempt will be retransmitted.  This value should not
              be  higher  than 255.  The default value is 5, which corresponds
              to approximately 180 seconds.

       tcp_timestamps (Boolean; default: enabled)
              Enable RFC 1323 TCP timestamps.

       tcp_tw_recycle (Boolean; default: disabled)
              Enable fast  recycling  of  TIME-WAIT  sockets.   Enabling  this
              option  is  not  recommended  since  this  causes  problems when
              working with NAT (Network Address Translation).

       tcp_tw_reuse (Boolean; default: disabled)
              Allow to reuse TIME-WAIT sockets for new connections when it  is
              safe  from protocol viewpoint.  It should not be changed without
              advice/request of technical experts.

       tcp_window_scaling (Boolean; default: disabled)
              Enable RFC 1323 TCP window scaling.  This feature allows the use
              of  a large window (> 64K) on a TCP connection, should the other
              end support it.  Normally, the 16 bit window length field in the
              TCP  header  limits  the window size to less than 64K bytes.  If
              larger windows are desired, applications can increase  the  size
              of  their  socket  buffers and the window scaling option will be
              employed.  If  tcp_window_scaling  is  disabled,  TCP  will  not
              negotiate  the  use  of window scaling with the other end during
              connection setup.

       tcp_vegas_cong_avoid (Boolean; default: disabled)
              Enable TCP Vegas congestion avoidance algorithm.  TCP Vegas is a
              sender-side  only  change  to  TCP that anticipates the onset of
              congestion by estimating the bandwidth. TCP  Vegas  adjusts  the
              sending  rate  by  modifying  the  congestion  window. TCP Vegas
              should provide less packet loss, but it is not as aggressive  as
              TCP Reno.

       tcp_westwood (Boolean; default: disabled)
              Enable   TCP   Westwood+   congestion  control  algorithm.   TCP
              Westwood+ is a sender-side only modification  of  the  TCP  Reno
              protocol  stack that optimizes the performance of TCP congestion
              control. It is based on end-to-end bandwidth estimation  to  set
              congestion  window  and  slow start threshold after a congestion
              episode. Using this estimation, TCP Westwood+ adaptively sets  a
              slow  start  threshold  and a congestion window which takes into
              account  the  bandwidth  used   at  the   time   congestion   is
              experienced.   TCP  Westwood+  significantly  increases fairness
              with respect to TCP Reno in wired networks and  throughput  over
              wireless links.

       tcp_wmem
              This  is  a  vector  of  3 integers: [min, default, max].  These
              parameters are used by TCP to regulate send buffer  sizes.   TCP
              dynamically adjusts the size of the send buffer from the default
              values listed below, in the range  of  these  sysctl  variables,
              depending on memory available.

              min  -  minimum size of the send buffer used by each TCP socket.
              The default value is 4K bytes.  This value  is  used  to  ensure
              that  in  memory pressure mode, allocations below this size will
              still succeed.  This is not used to bound the size of  the  send
              buffer declared using SO_SNDBUF on a socket.

              default  - the default size of the send buffer for a TCP socket.
              This value overwrites the initial default buffer size  from  the
              generic  global net.core.wmem_default defined for all protocols.
              The default value is 16K bytes.  If larger send buffer sizes are
              desired, this value should be increased (to affect all sockets).
              To   employ   large   TCP   windows,   the    sysctl    variable
              net.ipv4.tcp_window_scaling must be enabled (default).

              max  -  the  maximum  size  of  the send buffer used by each TCP
              socket.    This   value   does   not   override    the    global
              net.core.wmem_max.   This  is  not used to limit the size of the
              send buffer declared using SO_SNDBUF on a socket.   The  default
              value  is  128K  bytes.   It  is lowered to 64K depending on the
              memory available in the system.

SOCKET OPTIONS

       To set or get a TCP  socket  option,  call  getsockopt(2)  to  read  or
       setsockopt(2) to write the option with the option level argument set to
       IPPROTO_TCP.  In addition, most IPPROTO_IP socket options are valid  on
       TCP sockets. For more information see ip(7).

       TCP_CORK
              If  set,  don’t  send  out  partial  frames.  All queued partial
              frames are sent when the  option  is  cleared  again.   This  is
              useful for prepending headers before calling sendfile(2), or for
              throughput optimization.  As currently implemented, there  is  a
              200  millisecond  ceiling on the time for which output is corked
              by TCP_CORK.  If this ceiling is reached, then  queued  data  is
              automatically  transmitted.   This  option  can be combined with
              TCP_NODELAY only since Linux 2.5.71.  This option should not  be
              used in code intended to be portable.

       TCP_DEFER_ACCEPT
              Allows  a  listener to be awakened only when data arrives on the
              socket.  Takes an integer value (seconds), this  can  bound  the
              maximum  number  of  attempts  TCP  will  make  to  complete the
              connection.  This option should not be used in code intended  to
              be portable.

       TCP_INFO
              Used  to  collect  information  about  this  socket.  The kernel
              returns   a   struct   tcp_info   as   defined   in   the   file
              /usr/include/linux/tcp.h.   This  option  should  not be used in
              code intended to be portable.

       TCP_KEEPCNT
              The maximum number of keepalive probes TCP  should  send  before
              dropping the connection.  This option should not be used in code
              intended to be portable.

       TCP_KEEPIDLE
              The time (in seconds) the connection needs to remain idle before
              TCP  starts  sending  keepalive  probes,  if  the  socket option
              SO_KEEPALIVE has been set on this socket.   This  option  should
              not be used in code intended to be portable.

       TCP_KEEPINTVL
              The time (in seconds) between individual keepalive probes.  This
              option should not be used in code intended to be portable.

       TCP_LINGER2
              The lifetime of orphaned FIN_WAIT2 state sockets.   This  option
              can  be  used to override the system wide sysctl tcp_fin_timeout
              on this socket.  This is not to be confused with  the  socket(7)
              level  option SO_LINGER.  This option should not be used in code
              intended to be portable.

       TCP_MAXSEG
              The maximum segment size for  outgoing  TCP  packets.   If  this
              option  is  set before connection establishment, it also changes
              the MSS value announced to the other end in the initial  packet.
              Values greater than the (eventual) interface MTU have no effect.
              TCP will also impose its minimum and  maximum  bounds  over  the
              value provided.

       TCP_NODELAY
              If  set,  disable the Nagle algorithm.  This means that segments
              are always sent as soon as possible, even if  there  is  only  a
              small  amount  of  data.   When  not set, data is buffered until
              there is a sufficient amount to send out, thereby  avoiding  the
              frequent  sending  of  small  packets,  which  results  in  poor
              utilization of  the  network.   This  option  is  overridden  by
              TCP_CORK;  however, setting this option forces an explicit flush
              of pending output, even if TCP_CORK is currently set.

       TCP_QUICKACK
              Enable quickack mode if set or disable quickack mode if cleared.
              In quickack mode, acks are sent immediately, rather than delayed
              if needed in accordance to normal TCP operation.  This  flag  is
              not  permanent,  it  only  enables  a switch to or from quickack
              mode.  Subsequent operation of the TCP protocol will once  again
              enter/leave   quickack   mode  depending  on  internal  protocol
              processing and factors such as delayed  ack  timeouts  occurring
              and  data  transfer.   This  option  should  not be used in code
              intended to be portable.

       TCP_SYNCNT
              Set the number of SYN retransmits that TCP  should  send  before
              aborting  the  attempt  to connect.  It cannot exceed 255.  This
              option should not be used in code intended to be portable.

       TCP_WINDOW_CLAMP
              Bound the size of the advertised  window  to  this  value.   The
              kernel imposes a minimum size of SOCK_MIN_RCVBUF/2.  This option
              should not be used in code intended to be portable.

IOCTLS

       These following  ioctl(2)  calls  return  information  in  value.   The
       correct syntax is:

              int value;
              error = ioctl(tcp_socket, ioctl_type, &value);

       ioctl_type is one of the following:

       SIOCINQ
              Returns  the amount of queued unread data in the receive buffer.
              The socket must not be  in  LISTEN  state,  otherwise  an  error
              (EINVAL) is returned.

       SIOCATMARK
              Returns  true  (i.e.,  value  is  non-zero)  if the inbound data
              stream is at the urgent mark.

              If the SO_OOBINLINE socket option is set, and SIOCATMARK returns
              true,  then the next read from the socket will return the urgent
              data.  If  the  SO_OOBINLINE  socket  option  is  not  set,  and
              SIOCATMARK returns true, then the next read from the socket will
              return the bytes following the urgent data (to actually read the
              urgent data requires the recv(MSG_OOB) flag).

              Note  that  a  read  never  reads across the urgent mark.  If an
              application is informed of  the  presence  of  urgent  data  via
              select(2)  (using the exceptfds argument) or through delivery of
              a SIGURG signal, then it can advance up to the mark using a loop
              which   repeatedly   tests   SIOCATMARK   and  performs  a  read
              (requesting any number of bytes) as long as  SIOCATMARK  returns
              false.

       SIOCOUTQ
              Returns the amount of unsent data in the socket send queue.  The
              socket must not be in LISTEN state, otherwise an error  (EINVAL)
              is returned.

ERROR HANDLING

       When  a  network  error  occurs, TCP tries to resend the packet.  If it
       doesn’t succeed after some time, either ETIMEDOUT or the last  received
       error on this connection is reported.

       Some  applications  require  a quicker error notification.  This can be
       enabled with the IPPROTO_IP level IP_RECVERR socket option.  When  this
       option  is  enabled,  all incoming errors are immediately passed to the
       user program.  Use this option with care — it makes TCP  less  tolerant
       to routing changes and other normal network conditions.

NOTES

       TCP  has  no  real  out-of-band data; it has urgent data. In Linux this
       means if the other end sends newer out-of-band data  the  older  urgent
       data is inserted as normal data into the stream (even when SO_OOBINLINE
       is not set). This differs from BSD-based stacks.

       Linux uses the BSD compatible  interpretation  of  the  urgent  pointer
       field  by  default.   This  violates  RFC 1122,  but  is  required  for
       interoperability  with  other  stacks.   It  can  be  changed  by   the
       tcp_stdurg sysctl.

ERRORS

       EPIPE  The  other  end  closed  the  socket  unexpectedly  or a read is
              executed on a shut down socket.

       ETIMEDOUT
              The other end didn’t acknowledge retransmitted data  after  some
              time.

       EAFNOTSUPPORT
              Passed socket address type in sin_family was not AF_INET.

       Any  errors  defined  for ip(7) or the generic socket layer may also be
       returned for TCP.

BUGS

       Not all errors are documented.
       IPv6 is not described.

VERSIONS

       Support for Explicit  Congestion  Notification,  zero-copy  sendfile(),
       reordering  support and some SACK extensions (DSACK) were introduced in
       2.4.  Support for forward acknowledgement (FACK), TIME_WAIT  recycling,
       per  connection keepalive socket options and sysctls were introduced in
       2.3.

       The default values and descriptions  for  the  sysctl  variables  given
       above are applicable for the 2.4 kernel.

AUTHORS

       This man page was originally written by Andi Kleen.  It was updated for
       2.4  by  Nivedita  Singhvi   with   input   from   Alexey   Kuznetsov’s
       Documentation/networking/ip-sysctls.txt document.

SEE ALSO

       accept(2),  bind(2),  connect(2), getsockopt(2), listen(2), recvmsg(2),
       sendfile(2), sendmsg(2), socket(2), sysctl(2), ip(7), socket(7)

       RFC 793 for the TCP specification.
       RFC 1122 for the TCP  requirements  and  a  description  of  the  Nagle
       algorithm.
       RFC 1323 for TCP timestamp and window scaling options.
       RFC 1644 for a description of TIME_WAIT assassination hazards.
       RFC 2481 for a description of Explicit Congestion Notification.
       RFC 2581 for TCP congestion control algorithms.
       RFC 2018 and RFC 2883 for SACK and extensions to SACK.